<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    div{
      width: 100px;
      height: 100px;
      border: 1px solid red;
    }
    #d1{
        /*设置绝对定位,元素脱离文档流 后面的元素会顶上来*/
        position: absolute;
        /*通过left/right/top/bottom相对于窗口控制元素位置*/
        right: 30px;
        top: 20px;
    }
    #big{
        width: 200px;height: 200px;
        background-color: pink;
        margin: 100px 0 0 100px;

    }
    #big>div{
        width: 100px;height: 100px;
        background-color: blue;
        margin: 50px 0 0 50px;
        position: relative;/*设置相对定位 做参照物*/
    }
    #big>div>div{
        width: 50px;
        height: 50px;
        background-color: green;
        position: absolute;
        left: 0;
        top: 0;
    }

  </style>
</head>
<body>
<div id="big">
    <div>
        <div></div>
    </div>
</div>
<div id="d1">div1</div>
<div id="d2">div2</div>
<div id="d3">div3</div>
</body>
</html>
